Method for applying binding agents to fibrous webs

ABSTRACT

A method and system for applying a foamed binding agent to a fibrous web is disclosed. The invention is characterized by the use of an extruder having an upper lip extending 1/16 to 1/4 inch beyond the lower lip, and by the drawing of a vacuum beneath the applied foam, said characteristics being critical to the formation of a uniform foam layer atop the web. A second, more substantial vacuum is then applied beneath the web to essentially completely draw the binding agent into the web thereby providing substantial inter-fiber bonding.

This is a division of application Ser. No. 218,251, filed 12/19/80, nowU.S. Pat. No. 4,348,251.

FIELD OF THE INVENTION

This invention relates to the application of binding agents to dryformed, fibrous webs. More specifically, the present invention disclosesa method of application of said binding agents in the form of anextruded foam film, and, collaterally, to a system employed in carryingout the method.

BACKGROUND OF INVENTION

In current practice dry-formed fibrous webs are prepared by dispensing aloose continuum of fibers onto a moving foraminous support surface toform a loose web. Because the loose web as intitally laid lacksintegrity, a binding agent must be incorporated into the final webproduct. One method of incorporation admixes a particulate adhesive withthe fibers prior to their deposition onto the forming surface, theadhesive subsequently being activated by application of moisture and/orheat. In another method adhesive is sprayed onto the fibers as they areairborne within the distributor, or as they are in transit from thedistributor to the forming surface. Yet another technique is to dry-laythe fibers onto a moving foraminous wire, and subsequently to spray abinder solids bearing emulsion onto each side of the loose fibrous web,each application of the spray being followed by a drying step to atleast partially dry and perhaps partly cure the binder. If necessary,the bonded web is finally heated to completely cure the fibrous webproduct. An improved embodiment of this latter technique is described inthe commonly assigned pending patent application Ser. No. 108,022 filedDec. 28, 1979 entitled "Methods of Applying Bonding Materials OntoFibrous Webs."

The use of spray nozzles as described in U.S. Ser. No. 108,022, evenwith the utilization of vacuum to promote penetration, has severalinherent disadvantages. Nozzles tend to plug with binder solidsresulting in non-uniform coverage of the bonding material. While thisproblem is alleviated by larger sized nozzles, such nozzles causegreater usage of binder material. Further, the spray itself is notcompletely uniform in that is comprises discreet droplets of bondingagent, said droplets imparting to the web a mottled or dimpledappearance. At high nozzle discharge velocities, especially as such maydevelop with partially plugged orifices, the force of the droplets ontothe web can compromise web integrity notwithstanding the presence of thebonding agent. To ensure a completely covered web, the spray area mustextend beyond the outer edge of the web, including therefor portions ofthe carrier wire itself. This increases clean-up requirements and costssubstantially, as well as the quantity of bonding agent used. Finally,the airborne or atomized spray cannot be confined, and eventually coversother parts of the apparatus thereby aggravating the clean-up problem.

In contradistinction to the spray nozzle means above described, thepresent invention utilizes extruder means to apply a continuous film offoamed binding agent onto the web. Unlike U.S. Pat. No. 4,159,355granted to Kaufman, the applicator does not require an interiorconfiguration of special design, but does have critical geometricrelationships relating to its outlet dimensions.

SUMMARY OF INVENTION

It is an object of this invention to provide an improved method ofapplying a foamed layer of a binding agent to a dry laid, loose fibrousweb.

Another object of this invention is to provide a layer of binding agentto both sides of said web, at least one side being a foam layer.

A further object of this invention is to obtain uniform deposition of abinding agent onto each side of a fibrous web.

An additional object of the invention is to reduce the overall quantityof binding agent required on dry-laid fibrous webs.

Collaterally, it is an object of the invention to provide a system bywhich the application of the foamed binding agent may be carried out.

These and other objects will be more clearly understood upon aninspection of the drawings and a reading of the detailed description, asummary of which follows.

The fibrous web is formed by dispensing the individual fibers from adistributor header onto a moving foraminous support means or formingwire. Compaction of the web downstream of the header by means ofconsolidation rolls imparts some integrity to the web enabling transferto a second moving foraminous support, hereinafter the first carrierwire. The first (top) side of the web is treated with the binding agent,preferably a latex type material, at the first bond station. While anextruder type applicator laying down a film of binding agent foam ispreferable for this first bonding operation, loose webs having littleintegrity generally cannot withstand the force of a vacuum applied tothe underside of the web, said vacuum being a necessary feature of thefoam application system of the invention. For this reason conventionalspray means are often used except in those instances where web integritypermits the preferred use of foam. However, it is also preferred thatthe bulk of the binding agent be applied at the second bond station.Hence, the use of spray means at the first bond station does not detractfrom the advantages of the invention.

The once bonded web is then dried at least partially to eliminatetackiness resulting from the application of binding agent. Inverting ofthe web is accomplished by its transfer to a second carrier wire as isknown in the art. With the second side now in top position, the webpasses beneath an extruder which uniformly deposits a layer of foamthereon, the velocity of the foam being essentially equal to the speedof the second carrier wire. To ensure that the applied foam layerremains unitary with the web, it is critical that a slight vacuum bepulled just downstream of the extruder outlet, the vacuum being between0.1 to 1.0 inch of water. Substantial penetration of the foam layer intothe web to provide the internal bonding necessary to prevent plyseparation is obtained by drawing a second vacuum, between about 2 and15 inches of mercury, at a convenient downstream location. The twicebonded web is then dried and cured before being wound-up on a parentroll.

Notwithstanding the utilization of the first stage vacuum subsequent tofoam laydown, it was found that the foam still tended to separate,forming blotchy areas of uncovered web. However, a unitary foam layer ofuniform consistency was obtained by extending the upper lip of theextruder outlet a distance of from about 1/16 to about 1/4 inch.Conversely, an extruder having said lip extension did not provideuniform foam laydown without the application of said first stage vacuum.Thus, the system disclosed herein requires both to be present forsuccessful operation.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic view with a portion broken away of a dry laid webforming line, the foam application system of the present inventioncontained within the dashed lines.

FIG. 2 is a sectional view of one embodiment of an extruder dieavailable for use with the present invention.

FIG. 3 is a cross sectional view of the die along section 3--3 of FIG.2.

FIG. 4 is an enlarged detail view of the extruder die outlet orifice.

DETAILED DESCRIPTION OF INVENTION

FIG. 1 illustrates schematically a preferred embodiment of an apparatus,designated generally by numeral 10, for producing fibrous webs bonded onboth sides with a binding agent. Numeral 11 denotes the foam applicationsystem of the present invention, said system enveloped by the dashedlines. System 11 may also be adopted for use with pre-existing air laidforming lines, replacing thereby either or both spray bonding stationssubject to the below described constraints.

Referring to FIG. 1, loose fibrous web 18 is formed by depositing fibers12 from one or more distributor headers 14 onto an endless, foraminousforming wire 16, said wire being supported and transported by rollers17. Consolidation rolls 20, 22 compact the loose web 18 therebyproviding the web with some integrity. The web 18 is then transferred toa first foraminous carrier wire 25 by means of transfer wire 27. Vacuumbox 26 provides the upward force necessary to ensure that web 18 adheresto wire 27; vacuum box 30 removes the web from wire 27 and establishesit onto the carrier 25. While the forming wire 16 could be continued inlieu of carrier 25, the latter is often a coarser weave for ease ofcleaning residue binding agent therefrom. The exposed surface of web 18on carrier 25 is designated as the first side 28; the underlying surfaceas the second side 29.

The carrier 25 carries the web 18 beneath a first side 28 bondingstation generally indicated by numeral 31. In this instance the bondstation 31 is a spray applicator system 34, the binding agent 32 beingapplied to the surface 28. While not required with the spray typesystem, a vacuum box 36 can be used to provide greater penetration ofthe binding agent into the web, which, of course, gives the web evengreater integrity at this stage. The vacuum applied is slight, beingbetween about 0.1 to about 1.0 inches of water. Preferably, the vacuumbox 36 is directly below the spray system 34.

While an extruder which lays down a foam film of the binding agent isthe preferred applicator for bonding station 31, web characteristics atthis point in the apparatus 10 mitigate against its general deployment.The newly formed web, particularly webs of low basis weight and highbulk, such as those whose end products are tissue and towel sheets, donot have significant inter-fiber bonding. Hence, strength is minimal,even though the web may have been consolidated previously, as by rollers20, 22. Unlike spray applicators, the use of foam applicators requiresthat a slight vacuum be drawn beneath the web to prevent foamseparation, to ensure uniform distribution, and to provide minimumpenetration. Many webs do not have the requisite strength at bondstation 31 to allow the draw of said vacuum. However, those webs whichcan withstand the vacuum, generally between 0.1 and 1.0 inch of water,preferably below 0.5 inches of water, can be treated with a foam layerin lieu of the spray 32 at bond station 31.

Even if the preferred foam applicator is not employed at the firstbonding station 31, however, the substantial benefits of the inventionare still realized by the utilization of the foam applicator at thesecond bonding station. These benefits are obtained because the bulk ofthe binding agent is preferably dispensed at the second bonding stationwhere web integrity is substantially greater. In addition theconcentration of binding agent solids in the emulsion sprayed onto theweb at bond station 31 is preferably low, generally being between 15 and20% by weight. The disadvantages then; namely, poor distribution,plugging, and waste, are less pronounced at the first bond station 31.

The web 18 leaving the first bonding station 31 has now been providedwith a first binding agent layer, preferably, if consistent with webcharacteristics, of foamed binding agent, on surface 28. Because acertain amount of tackiness has been imparted to the web by thedeposition, the web is at least partly dried in drying means, here athrough air drier 38 of conventional design. When a water based latexbinding agent is used, the drier 38 is set at a temperature betweenabout 350° to about 380° F., the web being heated to between about 180°to about 200° F. The once-bonded, once dried web 18 leaves the drier atabout the processing temperature, and contains less than approximately5% moisture by weight, based on the total weight of the partially driedweb. As carrier 25 passes around roll 39, the web 18 is transferredinvertedly onto a second endless, foraminous carrier wire 40, surface 29now being on top thereof. Carrier 40 transports the web 18 through thesecond bonding station 42, the second drier means 47 and through curingmeans 48, before being wound up on parent roll 51. Curing means 48 maybe incorporated with drier 47, a cured web nevertheless being produced.It is also possible to transfer the web to a subsequent wire, said wirecarrying the web through the curing means 48.

As the web 18 passes beneath the bonding station 42, here including theextruder 43 of proper design as hereinafter described, a layer of foamedbinding agent 44 is applied to the surface 29. Initially, the foamcontains a high volume of air or other gaseous fluid, the blow ratiobeing between 10 and 25, preferably between 15 and 20. Blow ratio isdefined as the density of the unfoamed binding agent solution divided bythe density of the foamed dispersion. Almost immediately, the air woulddiffuse to the atmosphere causing partial collapse of the foam, which,but for the vacuum drawn in vacuum box 45, would cause foam separation,forming blotchy areas on the surface 29. However, by drawing the vacuum,maintained at a level of between 0.1 to 1.0 inch of water, preferablyless than 0.5 inch of water, at a point just downstream of extruder 43,foam integrity is retained. This level of vacuum is sufficient for onlyslight penetration of the foam into the web, no more than several tenthsof a millimeter. Hence, the interior of the web still containsessentially unbonded fibers. For this reason the web is passed over asecond vacuum box 46 located somewhat downstream of station 42. Thelevel of this vacuum is more substantial, being between 2 and 15,preferably between 5 and 10, inches of mercury, and allows the bindingagent to permeate the web essentially in its entirety.

A tracer dye placed in the foam of station 42 has shown that bindingagent has greatest concentration near the surface 29, as would beexpected, but also has appreciable quantities of binding agent evenproximate to the surface 28. Binding agent applied at bonding station 31was not tinted, and therefore could not account for the levels ofbinding agent proximate to surface 28.

It should be appreciated, as stated above, that the foam systemdesignated generally as numeral 11 could also be employed at bondstation 31 with certain webs in lieu of the spray means 34. However,should a foam system be used at bond station 31, it is not necessary,and is actually deleterious, to use a second vacuum box, for example,such as vacuum box 46 used at bond station 42. However, a vacuum boxequivalent to box 45 is required.

While the slight vacuum is deemed to be critical in preventing foamseparation on the surface 29 of the web, and while the second vacuumapplied by means of vacuum box 46 is important for substantial bindingagent penetration, yet another feature of system 11 has been found to beessential. This feature relates to the design of the extruder 43 used inapplying the foam, one embodiment being illustrated in FIGS. 2, 3 and 4.

FIG. 2 is a sectional side view of a "coathanger" type extruder die 100which can be used in applying the foam to web 18. However, otherextruder designs, which incorporate the relevant critical feature, mayalso be used. The die body comprises essentially cast members 101, 102,which are held together by screws 106, 107. The members 101, 102 arecast such as to provide flow passageway 109 along their adjacentinterior surfaces. Section 103 of member 10 is partially separatedtherefrom by channel 108, and forms with section 104 of member 102 thedie outlet orifice 105. The outlet orifice 105 can be varied byadjusting orifice adjusting screw 112, which also passes through stoplimit member 111, said member being held in place by screw 113. Foamfrom a reservoir (not shown) is admitted to the die 100 through inlet114, the foam passing through manifold 115 which is of the well known"coathanger" shape. This is seen most clearly in FIG. 3 a partial crosssectional view along section 3--3 of FIG. 2. Numeral 125 designates anextruder side plate. The flow passageway diameter can be varied byadjustment of the restrictor bar 121 located within aperture 122 bymeans of restrictor bar adjusting means 120, the screws 123 thereofadjusting bar 121 such that the pressure drops PAA', PBB', PCC' and PDD'are essentially equal. This ensure uniform velocities through thepassageway 109 at points A', B', C' and D'.

The critical feature of the extruder die 100 for the present inventionis the outlet orifice 105, shown enlarged in FIG. 4. It is essentialthat upper lip 116 extend beyond the edge 131 of lower lip 117 by atleast 1/16" and no greater than 1/4", the overlap dimension beingdesignated by letter L. Preferably this dimension is 3/32 inch. Byhaving the overlap L, the foam does not tend to roll or curl over theweb 18, and a uniform disposition can be obtained. Note that upper lip116 does not come in contact with the web 18.

While not critical, the angle of the orifice 105 to the web 18 ispreferably between 2° and 25°, and the width, W, of orifice 105 isfunction of web speed and foam volumetric flow rate as will behereinafter described. The height H of the die from the web is also notcritical, and may vary from about 1/4" to about 3", one inch beingpreferred.

Another critical feature of the invention is that the foam exiting theorifice 105 should be at a velocity approximately equal to the webvelocity. Foam velocity is a function of volumetric flow rate and crosssectional orifice area, hence, of foam density and binder add onamounts. Typically, add on rates are between 3 and 6 pounds bindersolids per ream (3,000 sq. ft.). The parameter most easily adjusted tobalance the foam and carrier velocities is the blow ratio, that is thedensity ratio of unfoamed binding agent (in solution) and foamed bindingagent. For example with a given add on rate of unfoamed binder solutionof known density and a fixed orifice area and carrier speed, the blowratio, ψ, can be calculated by the formula:

    ψ=(A.sub.o V.sub.c b)/W.sub.b

where

W_(b) =binder add on rate, lbs. unfoamed binder solution/min.

b=density of unfoamed binder solution, lbs./ft.³

A_(o) =orifice area, ft.², and

V_(c) =carrier velocity, ft./min.

Blow ratios are in the range of 10 to 25, and the other variables maywell have to be adjusted to ensure operation in this range. Generally,however, orifice width W is about 0.010 to 0.050 inches, preferablybetween 0.015 to 0.030 inches, thus further narrowing the selection ofoperating variations.

It has also been found that in addition to the savings obtained byelimination of waste, the foam bonded products actually require lessbinding agent to achieve a corresponding degree of strength as comparedto spray binding. The overall estimated savings in binding agent is inthe order of 10 to 30%, depending on the particular web to which it isapplied. It is believed that the reduction in binder usage isattributable to the uniformity of the foam layer onto the web, withpenetration into the web being likewise uniform. Thus, the interiorfibers are more completely bound and with less binding agent.Conversely, at the same add-on level CD wet tensile strength for a foambonded web is greater. Another advantage is that the concentration ofsolids contained in a foam system can be greater than in an unfoamedemulsion. Hence, less drying is required with concommitent savings inenergy costs.

Bonding materials which can be employed in the methods of this inventionare selected from the group consisting of binder solids-containingsolutions and emulsions and include any such bonding materials suitablefor bonding fibers. As used herein "fibers" includes natural fibers,most desirably paper-making fibers, especially those whose averagelength is generally about 1/4 inch or less, and/or synthetic fibers andfilaments and combinations thereof. Bonding materials such as polyvinylacetates, polyvinyl alcohols, starches, and dextrins can be employed.However, for webs comprised entirely or mostly of papermaking fibers, itis expected that the most satisfactory results wil be obtained withwater-based latex solids. Examples of such binding agents are acrylic,acrylic vinyl, styrene butadiene, and, most preferably, vinylacetate-ethylene polymer-containing emulsions, some of the latter ofwhich are commercially available for example, from Air Products andChemicals, Inc. under its trademarks, Flexac, e.g., Flexac 180;Flexbond, e.g. Flexbond 330; Airflex, e.g. Airflex 100HS, 456, 105 and120; Vinac, e.g., Vinac 880 and 881; and Vinar, e.g., Vinar 201 and 241.The preferred bonding material is Airflex 120 diluted with water.Preferably, to enhance foamability of the diluted latex emulsion aneffective amount of a suitable surfactant is added, generally between0.5 and 3.0% by weight. For use with the above mentioned binding agentmaterials, and in particular with Airflex 120, preferred surfactants arethe anionic re-wetting agents sold as Aerosol OT and Deceresol OT,registered trademarks of and commercially available from AmericanCyanamid Company. Optionally, a suitable catalytic agent such as sodiumbisulphate may be added to the system to promote cross-linking of thepolymer material.

A suitable foam is prepared simply by vigorous agitation and theintroduction of air as is well known in the art. Although the bindingagent applied at each bond location need not be the same, for bondingpapermaking fibers and forming fibrous web products such as wipers,preferably both agents are water-based latex types, and in mostinstances desirably they are of the same latex polymer type.

As obtained, water-based latex emulsions commonly contain from about 45%to about 60% by weight latex solids. As obtained, Airflex 120 mightcontain from about 45% to about 52% by weight latex solids, but usually,as received, it contains about 52% by weight latex solids.

The binder solids concentration of the second-applied binding materialmay be the same as that of the first-applied bonding material, butpreferably it is greater than that of the first-applied bondingmaterial. It has been found that fibrous webs comprised of softwoodKraft papermaking fibers previously spray bonded at bonding station 31,dried in drier 38, and transferred to carrier 40, and intendedultimately as web products of from about 30 to 50 lbs./rm. (3,000 sq.ft.) basis weight can be satisfactorily foam bonded at bond station 42with a foamed latex solids-containing emulsion Airflex 120, saidemulsion having a latex binder solids concentration from about 10% toabout 30% by weight, preferably between about 15 to about 25%, and mostpreferably about 20% by weight, based on the total weight of theunfoamed emulsion applied to the web, and a blow ratio of between 10 and25.

It can be stated generally then that the solids concentration of thefirst-applied binding agent, whether by foam or spray means, should begreat enough to provide the integrity the particular apparatusconfiguration requires to permit continuous transfers at commercialspeeds from carrier to carrier, yet not so great as to cause wastefulsurface accumulations of solids and consequent reductions as to solidspenetration and efficiency.

It is usually preferable to operate without a vacuum pressure at spraybonding station 31 because the vacuum tends to increase adherence of theonce-bonded continuum to carrier 25 and increase separation and transferproblems. It has been found, however, that a slight vacuum pressure of1.0 inch water or less may in certain applications be tolerable withinthe aforementioned solids concentration range of from about 15% to about20% by weight to control overspray and to hold the loose web on andprevent it from being lifted from carrier 25 by the spray velocityimparted from spraying means 34. Of course, should the web 18 be ofsufficient integrity to allow the use of this slight vacuum, foambonding means of system 11 may be employed thereat as mentioned above.

The total amount of binder solids to be included in the web product isto be as low as possible. Practically, within limits and in accordancewith this invention, many commercially acceptable high wet strength webproducts having CD wet tensiles of 750 grams per 3"×9" inch strips canbe obtained with low as well with high solid add-on levels. Aspreviously explained, high solids additions tend to accumulate on theweb surface and prevent good solids penetration, which fact is a key toobtaining both acceptable tensile strengths and solids efficiencies.

Of the total binder solids desired to be included in the web product,the first-applied and the second-applied bonding materials may add thesame percentage thereof, but preferably the first-applied bondingmaterial adds to the web a relatively minor or low percentage, and thesecond-applied adds the rest, or a relatively major or higher percentagethereof. Minor here means from measurable effective fiber bondingamounts to percent by weight, and major means from 50% up to less than100% by weight, based on the total weight of binder solids to beincluded in the fibrous web product.

For latex-bonded fibrous web products suitable for use asmoderate-to-heavy wipers the first-applied emulsion, whether by spray orfoam means, adds from about 20% to about 45% by weight, preferably about30% to about 40% by weight, of the total binders solids to be added onto or included in the final fibrous web product. The second-appliedfoamed emulsion adds on the rest, i.e., from about 55% to about 80%,preferably about 60 to about 70% by weight of the total binder solids tobe added onto or included in the fibrous web product. Although it hasbeen found that below about 20% by weight solids, the once-bonded webtends to be too weak to transfer from a bonding station 31 carrier to adrier 38 carrier, desirably the solids add-on is as low as practicableat bonding station 31 where little or no vacuum is applied, and solidsadd-on is as great as possible at bonding station 42 where a high vacuumis applied to increase solids penetration. Of course, the solids levelto be added at the respective first and second bonding stations and thetotal solids to be employed will vary depending form example on theapparatus and carrier configuration, its gentleness or severity withrespect to web handling and treatment, the bonding effectiveness of theparticular binder solids employed, and the basis weight, tensilestrength, liquid absorption, feel and performance characteristicsdesired of the web product.

The methods of this invention can be better understood from the examplesbelow comparing the conventional spray application method with method ofthis invention.

EXAMPLE I

In this example two fibrous webs of 100% wood pulp fibers were dry-laidon a forming wire, said webs having a basis weight of about 34 to 37lbs./ream (3,000 sq. ft.) on a binding agent free basis. Each web wasthen bonded with a spray of an Air Products Airflex 120 latex emulsionwhich had a 17% solids concentration by weight, dried in a through airdrier, and subsequently invertedly transferred to a second carrier wire.

The second surface of the first web was spray bonded, while the secondsurface of the second web was foam bonded in accordance with the presentinvention. The webs were then finally dried and cured in a through airdrier. In each instance, the binding agent was the same Airflex 120latex at a solids concentration (unfoamed) of 17% by weight. The foamhad a blow ratio of 15.

Just downstream of the foam applicator, a vacuum of 0.2 inches of waterwas maintained to ensure uniform coverage of the web by the foam.Further downstream of the foam applicator a vacuum of 10 inches ofmercury was drawn to pull the foam into the web. The foam extruder nipgap was 0.023 inches and the upper lip overlap was 3/22 inch. Line speedwas a constant 220 feet/minute for each web.

Binder solids add-on rates were selected so as to obtain a product webof comparable strength as indicated in the table below.

    ______________________________________                                                     Web        Web                                                                Portion 1  Portion 2                                             ______________________________________                                        Method used    Spray on side two                                                                          Foam on side two                                  Binder Solids Add-on                                                          (lbs./rm.):                                                                   Side 1         5.55         3.70                                              Side 2         5.55         5.51                                                 TOTAL       11.10        9.23                                              Basis weight (lbs./rm.)                                                                      44.9         46.3                                              CD Wet Tensile Strength                                                       (gms./3 in.)   1000         996                                               ______________________________________                                    

As is evident from the data, the overall binder solids add-on level wasreduced by about 17% from 11.10 to 9.23 lbs./rm. for webs of comparablebasis weight and strength. This savings appeared at the first bondstation, which decreases the drier load associated therewith, andalleviates significantly the cleaning problems referred to above.

EXAMPLE II

In this second example, the total add-on was equalized for each web todetermine the effect of the present invention on comparative CD wettensile strengths.

As in Example I the webs, now having a binder free basis weight of about31 lbs./rm. (3,000 sq. ft.), were dry-laid, the first-side being treatedwith a spray of Airflex 120 at 23% solids concentration by weight. Thesecond surface of the first web was spray bonded, and the second webfoam bonded as before. Here, however, the first vacuum pulled under theextruder was 0.6 inches of water, while the second vacuum was 5 inchesof mercury. The line speed was still 220 feet/minute and the lip overlapwas 3/32 inch, but the extruder nip gap was now 0.027 inches. The foamblow ratio was 25.

The tabulated results are:

    ______________________________________                                                     Web        Web                                                                Portion 1  Portion 2                                             ______________________________________                                        Method used    Spray on side two                                                                          Foam on side two                                  Binder Solids Add-on                                                          (lbs./rm.):                                                                   Side 1         2.6          3.0                                               Side 2         4.6          4.1                                                  TOTAL       7.2          7.1                                               Basis weight (lbs./rm.)                                                                      38.8         38.5                                              CD Wet Tensile Strength                                                       (gms./3 in.)   622          961                                               ______________________________________                                    

This data shows that the foam bonded web had a 35% higher tensilestrength at essentially equal binder add-on rates.

Although the invention has been described in detail above, it isintended that the invention be limited only in accordance with theclaims appended below.

We claim:
 1. A method of applying binding agent onto a second side of afibrous web whose first side had been bonded previously, the methodcomprising the steps of:(a) providing said once bonded fibrous web on amoving foraminous carrier, second side facing up, (b) applying to saidsecond side a foamed dispersion of a binding agent by extruder means,the extruder being characterized by an orifice outlet whose upper lipextends beyond its lower lip by between 1/16 and 1/4 inch, said foameddispersion being applied at an exit velocity approximately equal to thecarrier velocity, (c) drawing beneath said foraminous carrier a firstvacuum just downstream of the extruder outlet orifice, said vacuum beingbetween about 0.10 to 1.0 inch of water, whereby the foamed dispersionremains uniformly atop the web, the (d) drawing beneath the carrier asecond vacuum further downstream of the extruder means, said vacuumbeing about between 2 and 15 inches of mercury, whereby the foameddispersion is caused to penetrate said web.
 2. The method of claim 1wherein the concentration of binding agent solids in the unfoamedemulsion is between 10 and 30% by weight.
 3. The method of claim 2wherein the amount of binding agent solids applied to the second side ofthe web is between 50 and 80% of the total binding agent solids appliedto the web.
 4. The method of claim 3 wherein the foamed dispersion has ablow ratio of between 10 and 25, blow ratio being defined as the densityof the unfoamed binding agent emulsion to the foamed emulsion.
 5. Themethod of claim 1 wherein the concentration of solids in the unfoamedemulsion is between 15 and 25% by weight, the amount of binding agentsolids applied to the second side of the web is between 60 to 70% of thetotal solids applied to the web, the blow ratio is between 15 and 20,and the second vacuum is preferably between 5 and 10 inches of mercury.6. The methods of claim 4 or 5 wherein the preferred upper lip memberextension is 3/32 inch, the distance between the extruder outlet orificeand the web is between about 1/4 and three inches, and the angle betweenthe orifice and the web is about 2 to about 25°.
 7. The method of claim5 wherein the binding agent is a foamed water based latex emulsion.
 8. Amethod of applying binding agent to each side of a loose fibrous web,the method comprising the steps of:(a) providing said fibrous web on afirst moving foraminous carrier, first side facing up, (b) applying tosaid first side a binding agent emulsion, (c) at least partially dryingsaid once bonded web, (d) transferring said web to a second movingforaminous carrier, the web being inverted thereby, (e) applying to saidsecond side a foamed dispersion of a binding agent by extruder means,the extruder characterized by an orifice outlet whose upper lip extendsbeyond its lower lip by between 1/16 and 1/4 inch, said foameddispersion being applied at an exit velocity approximately equal to thecarrier velocity, (f) drawing beneath said foraminous carrier a firstvacuum just downstream of the extruder outlet orifice, said vacuum beingbetween about 0.10 to 1.0 inch of water, whereby the foamed dispersionremains uniformly atop the web, and (g) drawing beneath the carrier asecond vacuum further downstream of the extruder means, said vacuumbeing about between 2 and 15 inches of mercury, whereby the foameddispersion is caused to penetrate said web.
 9. The method of claim 8wherein binding agent is applied to the first side of the web by spraymeans.
 10. The method of claim 9 wherein a vacuum is drawn below thefirst carrier essentially beneath the spray means, said vacuum beingabout 0.1 to 1.0 inch of water.
 11. The method of claim 10 wherein theconcentration of the binding agent solids in the emulsion applied byspray means is between 15 to 20% by weight.
 12. The method of claim 8wherein the binding agent emulsion applied to the first side is a foameddispersion, said foam being applied thereto by extruder means, theextruder characterized by an orifice outlet whose upper lip extendsbeyond its lower lip by between 1/16 and 1/4 inch, said foameddispersion being applied at an exit velocity approximately equal to thefirst carrier velocity, and further comprising the step of drawingbeneath said foraminous carrier a first vacuum just downstream of theextruder outlet orifice, said vacuum being between about 0.10 to 1.0inch of water, whereby the foamed dispersion remains uniformly atop theweb.
 13. The methods of claim 10 or 12 wherein said vacuum drawn beneaththe first carrier is preferably less than 0.5 inches of water.
 14. Themethods of claim 13 wherein the vacuum drawn through the second carrierand just downstream of the extruder outlet orifice is preferably lessthan 0.5 inches of water.
 15. The method of claim 12 wherein theconcentration of the binding agent solids on an unfoamed basis appliedto the first side is about 15 to 20% by weight.
 16. The methods of claim11 or 15, wherein the concentration of binding agent solids on anunfoamed basis applied to the second side is about 10 to 30% by weight.17. The methods of claim 16 wherein the amount of binding agent solidsapplied to the second side is about 50 to 80% of all binding agentsolids contained in the web.
 18. The method of claim 17 wherein theconcentration of binding agent solids on an unfoamed basis applied tothe second side is about 15 to 25% by weight.
 19. The methods of claim18 wherein the blow ratio of foamed binding agent dispersion is between10 and
 25. 20. The methods of claim 19 wherein the binding agent is awater based latex emulsion, foamed where applicable.
 21. The methods ofclaim 8 or 12 wherein the preferred upper lip member extension is 3/32inch, the distance between the extruder outlet orifice and the web isbetween about 1/4 to three inches, and the angle between the orifice andthe web is about 2° to about 25°.